Thin ribbons of finely patterned foil, referred to as carrier tapes, are utilized in the semiconductor industry to assemble components such as integrated circuit chips into service packages. A tape has separate clusters of soft, inner leads to which respective chips are bonded and thereby carried for further assembly. Later the tape donates each cluster as a chip is removed and the inner leads are bonded to heavier, outer leads for service connection.
Each lead in a cluster on a tape should be precisely placed and the edges of the leads should be sharp and carefully delineated. Otherwise, registration of leads-to-pads on a chip or of leads-to-leads will be adversely affected in a bonding operation. A problem is that the placement of leads is becoming more precise and the leads in a cluster are increasing in number.
The fine and precise line work required to form such tapes is typically accomplished utilizing photolithographic techniques. A typical technique involves coating a conductive strip of foil, printing latent images in the coating, developing the images to expose unwanted foil and etching to remove such foil. Perhaps the most challenging step is the one of properly printing latent images of the leads in the coating.
A master mask containing a pattern of lead clusters is prepared, typically utilizing a laser machine controlled by a computer. The mask is mounted in a printing machine within a platen forming a path over which a strip will travel. The printer also includes strip handling apparatus, an intense light source and a system of controls.
A strip of conductive foil is coated with a light-sensitive material which is normally resistant to etchants. The coated strip is then indexed on a step and repeat basis over the path containing the mask. At each step a pattern is printed on a portion of strip, then the strip is indexed a precise distance and a next pattern is printed. A problem is to match the end of a preceding pattern to the beginning of a next pattern. Another problem is to align a series of such patterns so they follow a substantially straight reference datum. Such problems relate in part to the material of the strip and to the apparatus for handling such strip.
Preferably, such material is very thin which promotes good bonding and enhances temperature cycling tests of completed devices. Typically, high conductivity copper foil is utilized and annealled to a "dead soft" condition. Such foil is delicate and readily wrinkled, warped or otherwise distorted in handling.
The apparatus for handling thin strips includes reels, rollers and guides. A problem is that reels and rollers alone do not prevent a strip of foil from wandering about a straight line on a printing platen. The mask is typically set with respect to a reference centerline extending through the platen. Reels at each end of the platen are set to bias edges of a strip about the reference centerline. However, a strip of the type described is too weak to tolerate biasing pressure to guide in the manner described with the required precision. The guided edge of the strip has a tendency to crimp and tear and/or to misalign the residue portion of strip where printing takes place.
Accordingly, it is desirable to develop new and improved expedients for advancing and guiding a strip. It is particularly desirable to guide delicate strips of metallic foil such as thin strips of fully annealled copper utilized to make carrier tape. Such strips should be guided longitudinally and precisely along a given path, preferably by contacting along only one edge as a strip is advanced on a step and repeat basis. The strips and the tapes developed therefrom are further processed with facility on reels having wide flanges. Accordingly, any expedients for guiding should include restoring the strip to a condition amenable to winding the strip upon a reel.